According to the boards' settled case law, a failure to comply with the Art. 19(2) EPC provisions on the composition of opposition divisions is a substantial procedural violation which should normally lead to remittal of the case for fresh examination by a properly composed opposition division and reimbursement of the appeal fee (see e.g. T 251/88, T 939/91, T 382/92, T 476/95, T 838/02, T 1349/10, T 135/12, T 2536/16).
In some cases, e.g. T 251/88 and T 838/02, the parties have been invited to cite any reasons that might go against a remittal to the department of first instance. In both cases, the patent had been revoked by the opposition division. The board in T 825/08 followed this approach and observed that it might not always be strictly necessary under Art. 11 RPBA 2007 to remit a case if there were good reasons not to do so and both parties agreed. According to T 1700/10, violations of Art. 19(2) EPC should lead to a remittal regardless of the parties' position at least in situations where third parties are affected by the outcome of defective first-instance proceedings, as, in the case in hand, where the patent had been maintained in the opposition proceedings (see also T 1349/10). In such cases, the public too has an interest in the decision (T 234/11).
In T 2175/16 the board considered that a change in the composition of the opposition division between the holding of the oral proceedings and issuing the written decision to be a substantial procedural violation, which justified that the decision under appeal be set aside, the case remitted to the opposition division and the appeal fee reimbursed.